The present invention relates to a method for improving various properties of composite materials and a treatment agent therefor. More particularly, the present invention relates to a method for improving heat resistance against molten solder alloy and resistance against heat shock of a composite material composed of an inorganic reinforcing material such as glass fibers and mica flakes and a matrix resin such as epoxy resins and polyimide resins and a treatment agent of which the principal ingredient is a silane coupling agent.
Among various types of composite materials composed of an inorganic reinforcing material and a synthetic resin as the matrix, the most widely used ones in a variety of applications are those composed of glass fibers, e.g., glass cloths, glass tapes, glass mats and glass papers, or mica flakes as the inorganic reinforcing material and an epoxy resin, phenolic resin, polyimide resin, polyamide resin and the like as the matrix. It is a well established technology that the inorganic reinforcing material is subjected prior to compounding with the matrix resin to a pretreatment with a so-called silane coupling agent such as 3-aminopropyl triethoxy silane with the amino group as the functional group and 3-glycidyloxpropyl trimethoxy silane with the epoxy group as the functional group. The pretreatment of the inorganic reinforcing material with a silane coupling agent is undertaken in order to enhance the adhesion between the inorganic reinforcing material and the matrix resin with an object to improve the composite material prepared therefrom such as laminated plates with respect to the mechanical strength, electric properties, resistance against water either cold or boiling, resistance against chemicals and other important properties.
As is well known, the rapid progress and growth in the electric technology and industry in recent years have produced an increasing demand for laminated plates used as a substrate of printed circuit boards as prepared by using an epoxy resin or a polyimide resin as the matrix. Laminated plates for printed circuit board must be highly heat-resistant because the manufacturing process of printed circuit boards involves a step in which the substrate is immersed in a bath of molten solder alloy for the purpose of wiring. Conventional laminated composite materials prepared by using the above mentioned known silane coupling agents are disadvantageously poor in the heat resistance against molten solder slloy, presumably due to the large stress caused by curing at the interface between the inorganic reinforcing material and the matrix resin. An improved method in this regard has been proposed in Japanese Patent Publication 57-41771 in which the pretreatment of the inorganic reinforcing material is undertaken by using a cationic silane compound having a vinyl benzyl amino group or an aniline-substituted silane compound as the silane coupling agent. This method, however, is not completely effective in preventing blistering on the laminate, prepared using the silane coupling agent, when the laminate is immersed in a bath of molten solder alloy. In addition, the above mentioned cationic silane compound having a vinyl benzyl amino group is inherently poorly soluble in water so that preparation of a treatment solution requires a complicated process involving solubilization of the silane compound in water by a preliminary hydrolysis treatment to introduce hydrophilic silanol groups. The treatment solution of the solubilized silane compound is unstable and sometimes causes staining problems on the surface of the inorganic reinforcing material treated therewith, resulting in what appears to be of oil spots. The aniline-substituted silane compound is also defective due to low stability against exposure to light, which produces coloration so that the silane compound per se or the inorganic reinforcing material treated therewith must be kept in dark.
Further, laminated plates for printed circuit board are required to be improved in the resistance against heat shock because otherwise the heat shock caused by the immersion of the laminate in a bath of molten solder alloy may destroy the bonding between the matrix resin and the inorganic reinforcing material such as glass cloths or bonding between the surface of the laminate and the copper foil adhesively bonded thereto for forming an electric circuit, due to the stress caused by the difference in the thermal expansion coefficients.
Thus, eagerly desired is an improved treatment agent for the pretreatment of an inorganic reinforcing material used in composite materials of which the above mentioned problems relative to the heat resistance against molten solder alloy and resistance against heat shock of the composite material have been simultaneously and fully solved.